The invention relates to a drying section for a web drying machine for drying paper webs, or the like. In particular, the invention concerns air lock means which securely hold the web being dried and the feeder belt for that web to the principal drying cylinder. The invention will hereafter be described with respect to a paper web. It should, however, be understood that the invention is adapted for drying other moistened webs.
The drying section for a paper machine includes a plurality of drying cylinders therein. A paper web runs over these drying cylinders. An air permeable, usually endless, feeder belt, is applied directly to the exterior peripheral jacket or surface of at least one of the drying cylinders. Usually, the feeder belt is comprised of a porous material, e.g. a felt-like fabric material. The web to be dried passes around the outside surface of the feeder belt while the inside surface of the belt is in contact with the drying cylinder.
With known drying sections for paper machines, the web of paper sometimes undesirably lifts off the feeder belt as the web and feeder belt are being wrapped and deflected around the drying cylinder. This occurs due to pneumatic overpressures which are produced at the entrance side gusset defined at and before the line of initial engagement between the feeder belt and the overlying paper web, on the one hand, and the drying cylinder on the other hand. This also occurs as a result of centrifugal forces that are operative during the entire movement of the web around the drying cylinder. The lifting of the web off the drying cylinder produces blisters and wrinkles in the eventually dried paper web. Furthermore, heat emanating from the drying cylinder does not sufficiently pass through the feeder belt to the paper web if direct contact between the web and the feeder belt and between the feeder belt and the drying cylinder are not maintained. The intermediate air layer which may develop under the web prevents complete drying.
At high operating speeds, the paper web is subjected to a high elastic strain due to the centrifugal forces applied to the web as it wraps and deflects around the drying cylinder. If the web is not adequately adhered to the feeder belt and the drying cylinder, this elastic strain on the web can be expressed according to the formula R.sub.o =v.sup.2 /g, where R.sub.o designates that breaking length component which is required for compensating for the centrifugal force. At a machine speed of 25 m./sec., the minimal breaking length must amount to 64 mm., for example, to simply counteract the effects of centrifugal force. Moist paper webs being dried, however, only have a low wet strength. Therefore, it is important that a web be safely guided through the drying section in order to prevent blistering, wrinkling or perhaps even tearing of the web.